Mana Drain can target a spell that can’t be countered. When Mana Drain resolves, that spell won’t be countered, but you’ll still add mana to your mana pool at the beginning of your next main phase.

11/17/2017

If the target spell is an illegal target when Mana Drain tries to resolve, it won’t resolve and none of its effects will happen. You won’t get any mana.

11/17/2017

Mana Drain’s delayed triggered ability will usually trigger at the beginning of your precombat main phase. However, if you cast Mana Drain during your precombat main phase or during your combat phase, its delayed triggered ability will trigger at the beginning of that turn’s postcombat main phase.